Vince gave me these two URLs about film storage, and both places suggest
that film should be stored flat, not vertical. For instance,

Films should always be wound evenly, and never too tightly, with the
emulsion side out. Metal film storage cans or plastic boxes should be
uniform in size, stored flat, and never stacked more than twelve inches
high. Never put paper or any other loose material in the film storage can or
box.http://lcweb.loc.gov/preserv/care/film.html